The present invention concerns couplers for optical fibers. For coupling optical signals and for causing optical signals to branch over one or more optical fibers, one such coupler known in the art is referred to as the fused-elongated type coupler.
In FIG. 1, one example of a conventional fused-elongated type optical fiber coupler is shown. The optical fiber coupler 1 shown in the drawing is formed by aligning a section of each of two component optical fibers 2, mutually thermally fusing the aligned sections thereby forming a fused section, and then elongating or drawing out the heated fused section to form a fused-elongated region 3.
In the above described fused-elongated region 3, the diameter of each component optical fiber 2 is reduced as is the core of the component optical fibers 2. To the extent that diameters of the cores of the optical fibers are reduced, a proportionately greater fraction of the energy of the light propagated therein leaks through the cladding which surrounds the core. Also, to the extent that the component optical fibers 2 are drawn out and thereby elongated, the distance between the cores of adjacent optical fibers 2 is reduced, and due to this fact, the coupling between the propagated modes of the individual fibers becomes extremely great. In this way, the optical signal incident on one fiber 2 becomes split and is thus emitted at ports of both optical fibers 2. In the same way, separate optical signals incident on both component optical fibers 2 become combined.
With the conventional optical fiber coupler 1 described above, it is known that light incident on one optical fiber becomes proportioned over one or both optical fibers (referred to as coupling ratio) based on the wavelength of the incident light. For example, light incident on port A (or port B) of the optical fiber coupler 1 shown in FIG. 1 may be emitted by port C or port D, depending on the wavelength of the incident light. As illustrated by the graph of FIG. 2 which shows the percentage of light incident on port A of optical fiber coupler 1 that is emitted by port D as a function of the wavelength of the light incident on port A, the relationship between coupling ratio and wavelength is a sinusoidal function.
Optical fiber couplers do exist in which the fluctuation of coupling ratio as a function of wavelength is relatively flat. These optical fiber couplers having weak wavelength dependance for coupling ratio are known as wide wavelength range optical fiber couplers, an example of which is shown in FIG. 3. The conventional wide wavelength range optical fiber coupler 4 shown in FIG. 3 includes an optical fiber 5, a portion of which has undergone a preliminary elongation, and a conventional optical fiber 2. To form the optical fiber coupler 4, the initially elongated region of optical fiber 5 and a section of optical fiber 2 are aligned side by side and mutually thermally fused to form a fused section. The fused section thus formed is then elongated or drawn out to form a fused-elongated region 6. By virtue of the above mentioned preliminary elongation process, it is possible to create a propagation constant difference between the component optical fibers 2, 5. By so doing, it is possible to increase the mode coupling between the component optical fibers 2, 5 across the fused-elongated region 6, and thereby attain a desired coupling ratio. With such an optical fiber coupler 4, over a fairly wide range of wavelengths, a relatively level coupling ratio can be obtained, as is shown in FIG. 4. As with FIG. 2 for optical fiber coupler 1, FIG. 4 shows for optical fiber coupler 4, the percentage of light incident on port A that is emitted by port D as a function of the wavelength of the light incident on port A.
However, as FIG. 4 shows, for the conventional wide wavelength range optical fiber coupler 4, while relatively level compared with that of conventional optical fiber coupler 1, the coupling ratio as a function of wavelength does demonstrate a hump and thus, the conventional wide wavelength range optical fiber coupler 4 does not achieve truly flat, wavelength independent characteristics. The reason for this is thought to be that for the conventional wide wavelength range optical fiber coupler 4, even after elongation and reduction of the respective fibers' diameters, the central axes of each component optical fiber 2, 5 in the fused-elongated region 6 lie in the same plane. Thus, the modes which participate in coupling are thought to be limited to the fundamental modes of each core of each component optical fiber 2, 5. Based on light transmission coupling theory, it is believed that for mode coupling between only two modes, that the mode coupling waveform is such that perfectly flat wavelength characteristics cannot be attained.